This invention relates to electric motors and more particularly to direct current (DC) drive motors.
Many spacecraft applications rely on DC motors because of the relatively high output power available for a given motor size. Typically, these DC motors are used with a 28 volt DC power supply. The use of such low voltage power supplies requires that relatively high input currents be delivered to the motor through the motor brushes in order to obtain the desired output power.
However, such high input currents increase the current density in the motor bushes. This increase in current density can be detrimental to the useful life of the motor brushes. Since brush wear is also accelerated due to the near vacuum of the space environment, the need for reducing the current density of the brushes becomes more important.
Faced with the requirement for reducing current density in order to increase brush life, conventional DC motor design calls for increasing the cross sectional contact area of the brush either by increasing the brush size or by providing multiple brushes. Some attempts to reduce current density and increase reliability of conventional DC motors have led to compromises involving very large brushes of unusual and sometimes fragile proportions. Such compromises have sometimes succeeded in reducing high current densities only to result in high brush wear rates due to increased frictional resistance at the brush and commutator interface.
Another problem encountered in the near vacuum environment is the generation of electromagnetic interference that results from the electrical arcing that occurs when the brush breaks contact with each commutator bar during normal operation of the motor. This arcing also contributes to brush wear.
Still another problem encountered with conventional DC motor design is an undesirable torque ripple induced in the motor as the brush breaks contact with one commutator bar and makes contact with the next bar.
Another approach to reduce the foregoing problems for conventionally designed motors calls for increasing the number of coils in the armature windings with a corresponding increase in the number of commutator bars. Unfortunately, the increase in the number of bars requires a circumferentially narrower brush design to avoid shorting of too many bars. Moreover, the narrowing of the brush requires an extension of the brush dimension in the axial direction to maintain a desired current density which again can result in a fragile brush of abnormal proportions.
It is an object of this invention to provide an electric motor with longer brush life.
It is a feature of this invention to provide a smaller size electric motor.
It is still another feature of this invention to provide an electric motor with improved reliability.
It is a further feature of this invention to provide an electric motor having a reduced electromagnetic interference.
It is a still further feature of this invention is to provide an electric motor wherein the torque ripple is minimized.
In view of the foregoing discussion it would be advantageous, therefore, to remedy the above noted deficiencies. To this end the electric motor contemplated by this invention includes an armature having two opposed faces mounted on a shaft intermediate of the ends of the shaft. The electric motor further includes a pair of commutators mounted on the shaft. One each of the commutators is mounted adjacent to each of the faces of the armature. Each of the commutators includes a plurality of commutator bars, each of which has its circumferential width offset relative to the circumferential width of the commutator bars on the commutator mounted adjacent the opposite armature face.
These and other objects and features of our invention are pointed out in the following description and claims taken together with the accompanying drawing. It is to be understood, however, that the drawing is for purposes of illustration only and not a definition of the limits of the invention, the limits of the invention being determined by reference to the appended claims.